1. Field of the Invention
The present invention relates to an ion injection method and apparatus for injecting ions into a large-size processed article such as a semiconductor wafer at a large current and with low current density through an ion beam of low energy in a range of from the order of hundreds of eV to the order of tens of keV.
More specifically, the present invention relates to an apparatus in which a belt-like energy ion beam long in one direction is emitted from an ion source, intensively curved in the direction of the short side of means of a window/frame type magnet and subjected to mass spectrometric analysis so that only a desired kind of ions are injected into a wafer. Because the sectional area of the beam is large, there is characteristic that the current density is small while the current value is large.
2. Description of the Related Art
The ion injection apparatus is an apparatus in which an ion beam emitted from an ion source and having a predetermined amount of energy is injected into a subject through mass spectrometric analysis of the beam. Because the density n of ions to be injected and the area Q of a subject (such as an Si wafer, or the like) are fixed in advance, the total quantity nQ of required ions is determined. Because this is substantially equal to a value obtained by dividing the product of the current value I and the injection time T by the charge q (nQ=IT/q), the current I and the required injection time T are inversely proportional to each other. To enhance throughput, it is better to make the current value I large. Accordingly, the ion injection apparatus has a tendency that the current value is made larger and larger. Mass spectrometric analysis is used so that only a predetermined kind of ions are injected.
A conventional large-current ion injection apparatus is designed so that a narrow ion beam is subjected to mass spectrometric analysis by means of a magnet and then the ion beam is radiated onto a plurality of wafers put on a rotation target.
FIG. 18 is a schematic view of a conventional ion injection apparatus. The apparatus is designed so that an ion beam having a circular section or a square section is emitted from an ion source. The orbit of the beam is bent by means of a mass spectrometric analysis magnet. Then, the beam passes through a slit and strikes against wafers. The diameter of the ion beam is small so as to be in a range of from about 1 cm to about 5 cm. The reason why the beam is narrow is not in that a problem arises in the ion source but in that mass spectrometric analysis cannot be performed if the beam is wide. Although uniform magnetic field is generated by means of a magnet to thereby bend the beam, it is difficult to form uniform magnetic field in a wide range. Accordingly, a narrow beam is generated because of mass spectrometric analysis.
The Si wafer, which is a subject of ion injection, is however a large disk having a diameter of from 20 cm to 30 cm. Because the whole area of the wafer cannot be covered with the narrow ion beam, the beam is relatively displaced horizontally and vertically so that the ion beam is effectively radiated onto the whole area of the wafer. That is, the ion beam must be scanned on the subject. Although the ion beam may be moved left and right, the subject (wafer) is, in most cases, moved. The wafer is attached to a device called "target". A mechanism for moving the target while supporting the target is called "end station". Not only the beam density on the wafer must be uniform but also the beam must be incident onto a surface of the wafer at right angles. Therefor, a rotation target may be provided in the end station so that the wafer is attached to the rotation target. The target is displaced (Sy) to the direction (y-direction) of the surface, while it is rotated (Rz) at a high speed.
Assuming that the direction of the movement of the beam is the z-direction, then, for example, the rotation target is rotated around the z-axis and slowly moved translationally in the y-direction. Assuming that the subject is shaped like a circle with radius R, then the beam is injected into the whole surface of the subject when the rotation target is moved in the y-direction in a range of from -R to +R. The velocity v of the translational motion in the y-direction is not constant.
FIG. 19 shows a view of the relation between rotation (Rz) and translation motion (Sy) of the wafer in the prior art. In the conventional method, ions are injected evenly into the whole surface of the wafer by rotation and translational motion, but the beam moves at a linear speed of 2 ny.omega. on the circumference 2 ny of a circle if the beam strikes on the circumference at a distance v from the center of the wafer.
As v increases, the linear speed caused by rotation at a constant rotation angular speed increases. To compensate this, the velocity v of translational motion in the y-direction must be selected to be low. That is, the velocity v must become high in a neighbor of y=0 and must become low is a neighbor of y=.+-.R. The velocity v of translational motion in the y-direction must be controlled so that the relation v=c/.vertline.y.vertline. is satisfied. With respect to a range of y&gt;0, the motion is expressed by dy/dt=c/y and this is integrated to obtain a motion of y=(2ct).sup.1/2 and v=(c/2t).sup.1/2.
1. In a mechanism using a rotational target, a beam optical system is relatively simple. A narrow beam is drawn out of an ion source and subjected to mass spectrometric analysis by means of a magnet so that the beam can be applied onto the target as it is. The end station for supporting the rotation target is, however, complex.
2. For example, the disk on which the wafer is put must make a high-speed rotating motion and a translational motion simultaneously. The translational motion is not simple. The disk must move translationally at such a velocity (v=c/.vertline.y.vertline.) as inversely proportional to the position of the beam. For example, this has been proposed in .theta. D. Aitken, F. J. L. Robinson, M. T. Wauk, "Apparatus and Methods for Ion Implantation", U.S. Pat. No. 5,389,793.
3. .theta. V. M. Benveniste, "METHOD AND APPARATUS FOR ION FORMATION IN AN ION IMPLANTER", U.S. Pat. No. 5,554,857 has proposed an ion injection apparatus in which coils for controlling a quadrupole are added to a mass spectrometric analysis magnet so that convergence of a beam is controlled dynamically. In the proposed apparatus, however, the configuration of the magnet is very complex. Further, also the structure and motion of the end station are complex because a wafer is mounted on the target.
4. Further, in the conventional ion injection apparatus using a mass spectrometric analysis magnet and a rotation target, the beam current must be increased in order to enhance the ability of processing the wafer. If the current in a narrow local beam is increased, however, the current density becomes very high. The beam current density in the wafer is so high that a phenomenon of charging-up becomes remarkable. Accordingly, the beam current cannot be increased so sufficiently. That is, in order to avoid charging-up, the current cannot be increased sufficiently, so that the processing ability is limited.
5. To prevent charging-up, there has been proposed a further method in which low speed electrons are applied to an ion beam so that the electrons are trapped in the ion beam by electrostatic power thus to achieve space charge neutralization to thereby suppress the phenomenon of charging-up.
6. The proposed method can be achieved by attachment of a device called "plasma flood gun" to a neighbor of the wafer.
7. .theta. N. R. White, M. Sieradzki, A. Renau, "COMPACT HIGH CURRENT BROAD BEAM ION IMPLANTER", U.S. Pat. No. 5,350,926 has proposed an ion injection apparatus in which, after a laterally long divergent beam is converged laterally by means of a first magnet for generating magnetic field longitudinally, made to pass through a long hole so as to be subjected to mass spectrometric separation and diverged laterally, the laterally diverged beam is collimated by means of a second magnet for generating magnetic field longitudinally so that the laterally long collimated beam can be injected into the wafer. Because the vertically thin and laterally wide beam is made to pass through the gap, the gap of the magnet can be selected to be narrow. Because the beam is made to pass through the long hole after the beam is bent at 90 degrees by means of the first magnet, mass spectrometric separation is complete. The apparatus in which mass spectrometric separation is performed after the laterally long beam is ben laterally, however,
requires two magnet combinations. Accordingly, the configuration of the apparatus is complex, so that the adjustment of the beam distribution based on the adjustment of pole pieces of the first magnet is difficult.
8. There has been proposed a further method using a large-aperture ion source in which a large-area ion beam taken out from an ion source is moved straight directly so as to be injected into the wafer. Electrodes each having a large area and having a large number of holes are used as a lead-out electrode system such as acceleration electrodes, deceleration electrodes, etc. Mass spectrometric analysis, however, cannot be performed because the large-area ion beam cannot be bent by means of a magnet. Because mass spectrometric separation cannot be performed, there is a possibility that impurities may be injected into the wafer.
9. In short, any simple optical system has been never provided for providing mass spectrometric analysis of a large-current ion beam and transporting the beam.
10. It is an old practice to carry out deflection of ions on the basis of the magnetic field in the field of accelerator. Particularly in the field of high-energy physics, a magnet having a large gap has been produced to enhance the acceptance of a spectrometer used in reaction experiments for elementary particles.
.theta. J. Allinger, G. Danby, J. Jackson, A. Prodell, "High Precision Superconducting Magnets", IEEE Transactions on Nuclear Science, vol. NS-24, No. 3, June (1977)p 1299
.theta. T. Inagaki, Y. Doi, H. Hirabayashi, Y. Kato, K. Kawano, H. Sato, S. Sugimoto, K Takamatsu, E. Takasaki, T. Tsuru, H. Yoshimura, O. Asai, T, Satow, "Large Aperture Superconducting Magnet (BENKEI)", Cryogenics February (1984) p83
These show ideas for improving apparatuses attached to a huge accelerator. These are large-scale apparatuses for emitting and deflecting high-energy protons, electrons, positive electrons, etc. These are not small-scale industrial apparatuses. These cannot be applied to an ion injection apparatus, etc.